Pulmonary thromboembolism is an extremely
common preventable condition.
The most common preventable cause of death in hospital patients is
pulmonary thromboembolismز The vast majority of cases
are caused by emboli arising from thrombosis of deep leg veins
(calf, popliteal, femoral and iliac veins).
Diagnosis is notoriously
difficult as symptoms and signs are non-specific; most cases are
silent, resolving without detection.

The two main consequences of embolization to the pulmonary arterial
tree are an increase in pulmonary arterial pressure (which puts a
strain on the right side of the heart) and ischaemia of the lung,
with ventilated areas not being perfused by blood.
If 60% of the pulmonary vasculature is suddenly blocked, the heart
cannot pump blood through the lungs. There is cardiovascular collapse,
with electromechanical dissociation of the heart as it continues to
beat but develops no output. This pattern of blockage is known as
massive pulmonary embolism. Causing rapid death, it accounts for
about 5% of all cases of pulmonary thromboembolism .

Major pulmonary embolism occurs when there is blockage of
middle-sized pulmonary arteries. Patients commonly experience breathlessness.
Infarction of lung develops in only about 10% of such cases. It can
lead to haemoptysis and, if adjacent to the pleura, pleuritic chest pain.
It is not uncommon for patients to develop a subsequent massive
thromboembolism if untreated.

In about 85% of all cases of pulmonary thromboembolism there is blockage
of small peripheral vessels by small emboli (minor pulmonary embolism).
Patients may be asymptomatic or may experience breathlessness and pleuritic
chest pain as a result of small infarcts.
A very small number of patients develop recurrent minor pulmonary
embolism. There is blockage of many small peripheral arteries over
a period of many months by recurrent small emboli.
This leads to obliteration
of the vascular bed and right heart strain, causing pulmonary hypertension.
The main pulmonary artery contains a mass of red thrombus, a thromboembolus.
This originated by embolization from one of the femoral veins.

Embolism of material other than thrombus is less common.
Although thromboembolism is common, other material can form emboli
in unusual circumstances. For example, cholesterol embolism is
caused by fragments of fat derived from atheroma in
major arteries, and fat and bone marrow embolism usually form as
a result of severe trauma to bones, with fat and marrow from the medullary
cavity entering the venous system.
Tumour cell embolism is an
important method of spread of malignant tumour.
Air embolus is usually due to accidental pumping of air into the venous
system during intravenous injection or transfusion, and bubbles of gas
(nitrogen embolus) may form in the bloodstream in deep sea divers with
decompression sickness .

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